A line skirts are more of a cutting trick than a sewing trick. First, measure
from your doll's waist to wherever you want the knees to fall. Add a half
inch. This is your length. Then, measure around her waist. Divide by two
and add an inch. This is your width. Draw a rectangle on a sheet of paper
that measures your length by your width.

On the bottom of said rectangle, measure out an inch from each bottom edge.
Connect the new endpoint of the edges to the top edge to make a trapizoid.

Lost? Of course you are. Have an MSPaint diagram.

Pin it to your cloth and cut it out.

Now, fold your pattern in half. Fold your cloth, too. Place your pattern
with the straight edge about two inches from the fold and cut. Then,
cut straight down the fold. You should wind up with a long half of a trapizoid.
These are your backs.

Sew one angled edge of the back to each angled edge of the front of the skirt.
Don't sew the straight parts! You'll mess up your skirt!

Hem it. You should notice it starting to take a more conical, 3D shape, which
is what you want. Add an unfinished waist
band.